Imaging elements that contain coated on a support one or more radiation-sensitive silver halide emulsion layers to record imagewise exposure have been widely employed in both photography and radiography, since the imaging speeds usually far exceed those obtainable with other available radiation-sensitive materials. The emulsion layers contain radiation-sensitive silver halide grains, which are responsible for capturing electromagnetic radiation to form a latent image, and an aqueous processing solution permeable vehicle, which includes a peptizer for the silver halide grains and a binder to impart structural integrity to the layer or layers and adhesion to the support. Typically both the peptizer and the vehicle are comprised of a hydrophilic colloid, such as gelatin or a gelatin derivative. The radiation-sensitive silver halide emulsion layers as well as any other layers that, after imagewise exposure of the element, must be penetrated by aqueous processing solutions to produce a viewable image are typically coated as an aqueous dispersion on the imaging element support and then dried and hardened. Hardening allows the layers to retain their structural integrity when subsequently brought into contact with aqueous processing solutions, typically at elevated temperatures, but hardening is limited so that the layers remain processing solution permeable. The ability to construct radiation-sensitive silver halide emulsion layers and other, associated processing solution permeable layers using aqueous coating compositions is an important advantage in the manufacture of the imaging elements.
A general summary of photographic and radiographic imaging elements containing one or more silver halide emulsion layers, hereinafter referred to as silver halide imaging elements, is provided by Research Diclosure, Vol. 365, September 1994, Item 36544, and Research Diclosure, Vol. 184, August 1979, Item 18431. Research Disclosure is published by Kenneth Mason Publications, Ltd., Dudley House, 12 North St., Emsworth, Hampshire P010 7DQ, England.
It has been long recognized that magnetic recording layers can be usefully added to silver halide imaging elements to provide additional information. For example, a magnetic layer can be employed to record information relating to exposure and/or processing. Many, varied purposes can be served, depending upon the specific imaging application. For example, in motion picture film the magnetic recording layer can be used to provide a sound track, whereas in radiography the magnetic recording layer can be used to provide a permanent correlation between the image recorded and patient specific information. Specific citations of magnetic recording layers combined with silver halide photographic elements is provided by Research Disclosure Item 36544, cited above, XIV. Scan facilitating features, sub-paragraph (2).
Because of a variety of incompatibilities the clearly preferred location for a magnetic recording layer in a silver halide imaging element has been on the side of the support opposite that bearing the silver halide emulsion layer or layers--i.e., on the back side of support. Among the significant draw backs to integrating magnetic recording layers in silver halide imaging elements have been the following:
(1) The fact that magnetic recording layers have been typically coated using non-aqueous solvents. This has provided a disadvantage in manufacture, requiring separation of the magnetic recording layer and silver halide emulsion layer coating steps. Additionally, in many instances the resulting non-aqueous coatings have either lacked or exhibited limited permeability to the aqueous processing solutions, further dictating their back-side placement. PA1 (2) The magnetic recording layers have exhibited significant levels of optical density. In many instances magnetic recording layers are essentially opaque. In other instances the magnetic recording layers exhibit acceptable optical transmission in one region of the spectrum, but not in another. Blue absorption by the magnetic recording layers has been a particular drawback. PA1 (3) The magnetic recording layers have been noted to elevate image granularity when positioned to intercept exposing radiation. The metal oxide (usually ferric oxide) magnetic particles that store magnetic information with the magnetic recording layers exhibit much higher refractive indices than the organic binders in which they are coated and, hence, can contribute significantly to light scattering, depending on their sizes and coating concentrations. PA1 (4) It is generally recognized that the photographic properties of silver halide emulsions are vulnerable to metal contamination. Keller Science and Technology of Photography, VCH Publishers, New York, 1993, at page 58 states: PA1 Even the lowest level of impurities in an emulsion can markedly impair the photographic result. Process equipment, peripheral equipment, and all raw materials used therefore meet strict cleanliness and purity requirements. PA1 (a) radiation-sensitive silver halide grains, PA1 (b) an aqueous processing solution permeable vehicle, PA1 (c) from 0.1 to 10 mg/dm.sup.2 of magnetic particles having a major axis less than 1 .mu.m and preferably less than the mean equivalent circular diameter (ECD) of the silver halide grains, and PA1 (d) based on the weight of the magnetic particles, from 10 to 200 percent of an amphipathic dispersant for the magnetic particles having a hydrophilic/lipophilic balance number of at least 8. PA1 B=Blue Recording Layer Unit, PA1 G=Green Recording Layer Unit, PA1 R=Red Recording Layer Unit, and PA1 S=Support. PA1 Item 36544 PA1 Vol. 370, February 1995, Item 37038 PA1 Item 18431 PA1 Item 36544 PA1 Item 37038
Appropriate filtration units must deliver air free of both solid and gaseous contaminants, especially hydrogen sulfide. Water is usually treated on ion-exchange resins and must not contain any reducing agents. The specification of silver nitrate and halides is stringent, especially for heavy-metal impurities: the concentration of iron, copper, and lead must be &lt;1 ppm. PA2 IV. Chemical Sensitization PA2 V. Spectral sensitization and desensitization PA2 XV. Emulsions, including particularly, PA2 V. Cross-Over Exposure Control PA2 VIII. Absorbing and scattering materials PA2 XIII. Filter and Absorber Dyes
With so many disadvantages to be reduced or eliminated by being able to coat magnetic recording layers of acceptable specular transmittance like other aqueous processing solution permeable layers coated with silver halide emulsion layers, it is not surprising that a few attempts to achieve this objective have been reported along with other, undemonstrated suggestions of such coating possibilities.
Namikawa et al Canadian Patent 686,172 shows that a magnetic recording layer may be transparent to visible light when it contains low concentrations of magnetizable particles. According to this patent, such a layer is coated over a layer containing descriptive material which allows a user to simultaneously hear and see certain subject matter. However, this patent points out that the electromagnetic characteristics, i.e., the magnetic recording and reproducing characteristics, of such a layer are inferior to those of conventional magnetic layers as a result of the very low concentration of-magnetizable particles.
Krall U.S. Pat. No. 3,782,947 discloses a photographic product which carries magnetic particles distributed across the image area of the product at any location, including in front or back side separate layers or in the base or a radiation-sensitive emulsion layer. A variety of silver and non-silver radiation-sensitive materials are disclosed. In every instance in which Krall employs a silver halide emulsion the magnetic recording layer is located on the back side of the support, indicating Krall's awareness of the art-recognized incompatibility of silver halide grains and iron particles.
Yubakami et al U.S. Pat. No. 4,758,275 sets out to overcome the brownish color of magnetic particle dispersions by employing an organic dispersion medium and a colorant. Numerous problems associated with the use of aqueous dispersions of magnetic particles are identified.
Audran et al U.S. Pat. No. 4,279,945 discloses a process of preparing magnetic recording elements containing a recording layer that is transparent over a portion of the visible spectrum. In the Figure transmission is shown to be near zero in the visible wavelength range of from 400 to 500 nm and less than 20 percent at 550 nm, but above 60 percent at 650 nm. Magnetic particle dispersions in organic liquids are employed. The magnetic recording layer is taught to be coated on the back side of the support or over a silver halide emulsion layer. Audran et al further suggests coating over the silver halide emulsion layer and also on the back side of the support.
Bishop et al U.S. Pat. No. 4,990,276 discloses a dispersion consisting essentially of magnetic particles, a dialkylester of phthalic acid, and a dispersing agent. The dispersion is also disclosed to be diluted with organic liquids.
None of the elements of the type described in the above-cited patents have overcome the problems identified above and none of the elements have enjoyed widespread commercial success.
By contrast, Nair et al U.S. Pat. No. 5,457,012 has successfully demonstrated a stable fine solid particle aqueous dispersion which addresses each of problems (1) to (3) above. Nair et al discloses an aqueous medium containing dispersed magnetic particles and an amphiphatic dispersant having a hydrophilic/lipophilic balance of at least 8. Nair et al discloses that the magnetic particles of less than 1 .mu.m in mean size and coating densities of up to 10 mg/dm.sup.2 provide acceptable transparency for use in silver halide imaging elements. Nair et al teaches to coat a magnetic recording layer in the support, on the back side of the support, or on the front side of the support over or between silver halide emulsion layers.